An organic light-emitting device has features such as favorable a merit of visibility for a self light-emitting, planar light emission to make a sharp thin, lightness in weight, being driven at a low voltage, and being capable of multicolor and full-color displaying, has got a lot of attention as a next-generation display device, and has been expected to be applied to flat panel displays.
It is a fundamental structure in a current organic light-emitting device that Tang et al. of Eastman Kodak Company reported in 1987 (refer to Non-Patent Document 1).
(Non-Patent Document 1)
C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)
In Non-Patent Document 1, a sufficient luminance of 100 cd/m2 at 5.5 V is attained by making an uniform extra-thin film with a thickness of approximately 100 nm as an organic thin film, selecting an electrode material to make a carrier injection barrier against the organic thin film smaller, and introducing a heterostructure (a two-layer structure).
Further, the organic light-emitting device in Non-Patent Document 1 in a manner has an origin of an idea of a functional separation of layers, where a transport of a hole is carried out by a hole transporting layer and a transport of an electron and light emission are carried out by an electron-transporting light-emitting layer. The idea of this functional separation is further developed into an idea of a double heterostructure (a three-layer structure), where a light-emitting layer is interposed between a hole transporting layer and an electron transporting layer (refer to Non-Patent Document 2).
(Non-Patent Document 2)
Chihaya ADACHI, Shozuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films with Three-Layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2, L269-L271 (1988)
The functional separation of layers has an advantage that it becomes unnecessary to make one type of organic material have various functions (such as a light emission, a carrier transport, and a carrier injection from an electrode) at the same time so that a molecular design of a material to be used for a device is allowed to have a wide range of flexibility (For example, it becomes unnecessary to search a bipolar material by constraint). Then, an advantage of a material is effectively utilized to enable an improvement of a luminous efficiency easily by combining each of various materials such as a material that has a favorable light-emission characteristic, a material that is superior in transporting a carrier, a material that is superior in injecting a carrier. In this way, many of current organic light-emitting device have a multilayer structure.
Some of multilayer organic light-emitting devices utilizing this functional separation of layers have a structure where an electron transporting material into which a hole is hard to inject due to a large ionization potential, that is, a hole-blocking electron transporting material (hereinafter, referred to as “a hole blocking material”) is used as a hole blocking layer.
As described above, the hole blocking layer has a large ionization potential, and it is quite difficult to inject a hole into the hole blocking layer. Therefore, a hole carrier injected into a light-emitting layer is not allowed to enter the adjacent hole blocking layer to be kept in the light-emitting layer. Then, a hole or electron density is increased in the light-emitting layer to recombine the carriers efficiently, and the organic light-emitting device can be highly efficient by this efficient recombination of carriers.
In addition, since this hole blocking function makes it possible to control a recombination region of carriers, it is also possible to control a color of a light emission of the organic light-emitting device.
For example, there is a report that the hole transporting layer is successfully made to emit light by inserting a hole blocking layer between a hole transporting layer and an electron transporting layer (refer to Non-Patent Document 3).
(Non-Patent Document 3)
Yasunori KIJIMA, Nobutoshi ASAI and Shin-ichiro TAMURA, “A Blue organic Light Emitting Diode”, Japanese Journal of Applied Physics, Vol. 38, 5274-5277 (1999)
Here, a mechanism of keeping a hole in the transporting layer by the introduction of the hole blocking layer and recombining carriers in the hole transporting layer is working.
In addition, as an application example of a hole blocking layer, there is an example of being applied to a triplet light-emitting device (refer to Non-Patent Document 4).
(Non-Patent Document 4)
M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, 4-6 (1999)
Although a triplet light-emitting device is an efficient technique for a high-efficiency organic light-emitting device, a hole blocking material is an important key since light cannot be emitted efficiently when no hole blocking layer is used.
In this way, a hole blocking material is a material that can be used quite efficiently. However, a material that has a highly electron-transporting property and a favorable hole blocking property is limited to a large degree under present circumstances. Although few examples thereof include, for example, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter, referred to as “BCP”) that is used also in Non-Patent Document 3 and Non-Patent Document 4, in addition, 1,3-bis [(4-tert-buthylphenyl)-1,3,4-oxadiazole] phenylene (hereinafter, referred to as “OXD-7”), and 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter, referred to as “TAZ”), a deposited thin film is severely crystallized as for every material, and reliability is adversely affected significantly in the case of being applied to an actual device. For example, a triplet light-emitting device using BCP has a device life (a half life of a luminance) of 170 hours at an initial luminance of 500 cd/m2, which is really far from a level in practice use (refer to Non-Patent Document 5).
(Non-Patent Document 5)
Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo WAKIMOTO and Satoshi MIYAGUCHI, “High Quantum Efficiency inorganic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese Journal of Applied Physics, Vol. 38, No. 12B, L1502-L1504 (1999)
On the other hand, in order to overcome this point, there is an example where (2-methyl-8-quinolinolato)-(4-hydroxy-biphenylyl)-aluminum (hereinafter, referred to as ‘BAlq’) that is a material that is hard to crystallize is applied to a hole blocking layer to manufacture a triplet light-emitting device (refer to Non-Patent Document 6).
(Non-Patent Document 6)
Teruichi WATANABE, Kenji NAKAMURA, Shin KAWAMI, Yoshinori FUKUDA, Taishi TUJI, Takeo WAKIMOTO and Satoshi MIYAHUCHI, “Optimization of driving lifetime durability in organic LED devices using Ir complex”, Proceeding SPIE, vol. 4105, 175 (2000)
In Non-Patent Document 6, it is reported that while a half life of a luminance (an initial luminance of 600 to 1200 cd/M2) is 700 hours or less in the case of using BCP as a hole blocking layer, a half life of a luminance (an initial luminance of 570 cd/m2) is remarkably extended to about 4000 hours in the case of using BAlq.
However, as compared to a device using BCP, a device using BAlq has an efficiency adversely decreased considerably. This means that, as compared to BCP, BAlq is inferior in hole blocking property while being hard to crystallize (a quality of film is stable).
Further, in a triplet light-emitting device, data is reported where various hole blocking materials are used to compare a half life of a luminance in each device (refer to Non-Patent Document 7).
(Non-Patent Document 7)
Raymond C. KWONG, Matthew R. NUGENT, Lech MICHALSKI, Tan NGO, Kamala RAJAN, Yeh-Jiun TUNG, Michael S. WEAVER, Theodore X. ZHOU, Michael HACK, Mark E. THOMPSON, Stephen R. FORREST and Julie J. BROWN, “High operational stability of electrophosphorescent devices”, Applied Physics Letters, Vol. 81, No. 1, 162-164 (2002)
In Non-Patent Document 7, TPBI that has a quite highly hole blocking property is used, and it is proven that a half life of a luminance of a device that has a quite high luminous efficiency is a several orders of magnitude lower as compared to a half time of a luminance of a device using BAlq.
As these reports, it has never reported that a half life of a luminance is long to be able to obtain high reliability in a device that has a highly hole blocking material applied to a hole blocking layer. A cause for this is that crystallization of a material that is used for a hole blocking layer is severe as cited in the example of BCP. In other words, stability of a quality of film in a thin film state is lacking, and therefore, high reliability cannot be obtained.
Accordingly, a hole blocking layer that has a highly hole blocking property and stability of a quality of film is desired.